It is well known that as a borehole is drilled, it is necessary to assure that the fluids found in the virgin rock or formation are not permitted to flow uncontrollably into the borehole. In extreme situations, where the formation fluid is a gas, either in its gaseous or dissolved state, incursions of the formation gas into the borehole has the effect of diluting the column of drilling mud, thereby significantly reducing bottom hole pressure and increasing the flow of formation fluids from the rock into the borehole. If this process, which tends to feed on itself, is permitted to continue, an event called a "blowout" may occur. Blowouts are undesirable not only due to the loss of the valuable formation fluids, such as hydrocarbon oil or gas, but more importantly, uncontrolled flows of formation fluids at the earth's surface is a source of pollution and, when the fluids include hydrocarbons, are likely to be ignited to produce a burning well.
As a result of this scenario, it is conventional to drill the borehole with a drilling mud whose density, (mud weight), is controlled in order to assure that there is little or no chance that the formation fluids can flow into the borehole. This is accomplished by providing a drilling mud that produces a hydrostatic pressure at the bottom of the well which exceeds the pore pressure of the fluids in the rock formation. The disastrous consequences of a blowout usually cause the driller to be conservative and to specify a drilling mud weight that is calculated to guarantee that bottom hole mud pressure exceeds by quite a margin the expected formation pore pressure. Unfortunately, there has, till now, not been available a technique for reliably determining the formation pore pressure while the borehole is being drilled. Thus the driller is likely to provide a large pressure overbalance (i.e. the difference between bottom hole mud pressure and the formation pore pressure) since the drill bit may enter an overpressured formation at any time. Drilling with a large pressure overbalance may be detrimental in that it tends to increase the "hardness" or Formation Strength of the rock thereby reducing drilling rate and, in extreme cases, it may exceed the fracture strength of the rock to thereby cause formation damage. By "Formation Strength" is meant the resistance to borehole excavation posed by the geological formation to the drill bit while the borehole is being drilled.
As sediments are buried by the deposition of materials above them, the downward pressure exerted on the materials being buried by those above cause the sediments to compress thereby reducing the pore space found between the grains of the sediment. Under normal conditions of compaction, the fluids contained in the pore space are expelled from the sediments and flow through neighboring permeable formations. In this situation, the weight of the overburden is born by the matrix of the sediments and the pore pressure is determined by the hydrostatic pressure of the fluids at that particular depth. If, however, the fluids are not permitted to flow out of the sediments that are being compressed, the pore volume, rather than decreasing, will remain essentially the same and the pressure of the fluids in the formation will provide partial support of the downward pressure exerted by the overburden. The overburden is then supported both by the rock matrix and the trapped, highly pressurized formation fluids within the pore space. Such is likely to be the situation where long columns of clay or silt sediments, which usually have a small permeability, are buried rapidly, thereby not permitting the water to escape.
With this explanation, it can be understood that fluid pressures in formations which exceed those resulting from only considerations of hydrostatics are related to an "excess porosity" as compared to those formations at the same depth which were formed in a manner which permitted the formation fluids to escape and the formation matrix to compress with a normal pore space reduction. For the purposes of this application, the excess porosity will be called overpressure porosity, phi.sub.op, and the fluid pressure in the formation will be called the pore pressure, PP. Also, for the purposes of this application, the porosity to be expected from non-exceptional formations will be called the "effective porosity", phi.sub.ef, and the portion of the pore space filled by water will be called the "water porosity", phi.sub.w.
Many attempts in the past have been made to determine the pore pressure by various techniques, most of which rely on the comparison of a measured parameter to an expected trend in that parameter attributable to increasing burial depth and decreasing porosity. Take, for example, trends in sonic transit time, (delta t), which is normally expected to exhibit a reducing trend with depth. In addition, it is known that formations having larger values of porosity tend to drill more easily, or to have smaller Formation Strength, than formations with smaller porosities. However, no prior attempts have been made to separate formation porosity into a "normal", or effective porosity, and an "exceptional", or excess porosity from which may be determined a value of formation pore pressure in order to detect overpressure conditions.